The present invention relates to color printing, and in particular to color printing in which a small set of colorants, such as inks, are placed on a sheet in a manner to create a large range of apparent colors. The present invention is most useful in digital printing devices, such as xerographic or ink-jet printers.
Digital halftoning is the process of converting a continuous tone image to bi-level. Many output devices, including many printers and some cathode ray tube (xe2x80x9cCRTxe2x80x9d) and liquid crystal display (xe2x80x9cLCDxe2x80x9d) based devices are intrinsically bi-level. In other words, the process only prints or displays a dot or the process does not print or display a dot. Thus, a variety of geometrical patterns are created such that a group of dots and blank areas represent the continuous tone image as closely as possible. Because the halftoned image is only an approximate representation of the continuous tone image, there are differences between the continuous tone image and the halftone image. Those areas of the halftone pattern which do not match the original image are noise or error. An objective of much research in digital halftoning is reducing the amount of visible noise.
There are two broad classifications of digital halftoning: conventional, passive halftoning and active halftoning. Conventional, passive halftoning yields an appearance that is similar to that provided by classical analogue processes developed before digital techniques were available. It is most appropriate for devices that cannot display isolated pixels. Active halftoning typically implements some type of error diffusion to produce an image having a more pleasing appearance. Images created using error diffusion tend to have noise at higher and, hence, less visible spatial frequencies. Error diffusion is most appropriate for devices that can display isolated pixels.
Digital printing devices implementing halftoning technology, such as electrophotographic laser printers and ink-jet printers, are well known. In color digital printing devices, a small set of primary colorants, typically black, yellow, magenta, and cyan, are selectably placed in different areas on a print sheet. Small areas of each primary colorant are then optically blended together to create a large range (i.e., gamut) of colors which would be apparent to an observer.
Error diffusion allows a large gamut of colors to be obtained from a small number of colorants in a halftoned image. In order to print a small area of a desired source color using error diffusion, the source color is located in color space relative to the locations of the primary colorants, such as cyan, magenta, yellow, and black. The colorant which is closest to the target color in color space is then selected for one pixel. However, the error, essentially meaning the Euclidean distance in color space between the source color and the selected colorant, is recorded, and is in effect distributed or diffused to the image data of subsequent neighboring pixels. In brief, this diffusion of each error to neighboring pixels influences the decision of which primary colorant to use in those neighboring pixels. The overall effect is, over a reasonably large number of pixels, an optical blend resulting in the desired source color.
Recently, particularly in the technology of ink-jet printing, there has been developed a hardware option in which selectably available colorants are provided beyond the usual pure primary colorants of cyan, magenta, yellow, and black. For example, some designs may include colorants of additive colors, such as red, blue, and green, in addition to the subtractive colors of cyan, magenta, and yellow. Other designs may include colorants representing a lighter or diluted version of another primary color, such as a light-cyan, which is 50% lighter than regular-cyan. The use of such additional colorants can enhance and/or enlarge the available gamut associated with a particular apparatus. One particular additional colorant, which will be the subject of the embodiment of the present invention described below, is, in addition to a pure black K colorant, a grey LK colorant. The grey LK colorant is a 50% dilution of black K ink. Selectable use of the grey LK ink will, of course, be helpful in the creation of monochrome halftones, such as black-and-white photographs, and also for the creation of non-saturated colors. Other additional colorants which will be discussed include light-cyan LC, light-magenta LM, and light-yellow LY.
These additional colorants, such as light-cyan or grey, are considered intra-gamut colorants. More specifically, the pure colorants such as cyan or magenta define a gamut and are, therefore, disposed at the corners of a cube 10 representing a color space of the gamut. The intra-gamut colorants, on the other hand, are disposed within the color space of the gamut. In other words, the intra-gamut colorants are along the edges, on the faces, or within the cube 10. While this is useful for obtaining accurate representations of colors, such as pastels, which are near the white or grey areas of a gamut, use of such intra-gamut colorants can interfere with the error-diffusion colorant selection process.
For example, an error-diffusion selection process may occasionally decide that an intra-gamut colorant such as grey LK, or a mixture of on-edge colorants such as light-cyan LC plus light-magenta LM (i.e., a color on one of the faces of the color cube), is desirable for a particular pixel in an image. In either of these situations, the resulting error from selection of the intra-gamut color, when diffused to influence the selection of colorants for neighboring pixels, may require the selection of colors which are out of the gamut. These out-of-gamut colors would be physically impossible to obtain with the available colorants. In other words, selection of a colorant either inside the gamut or on the face of the gamut may lead to errors which require selection of colorants outside the gamut, which are not available. Consequently, artifacts from large errors may occur.
To illustrate the drawback of the prior art system, FIG. 1 illustrates a cube 10 representing a color space CS1. The color space characterizes a gamut indicating a three-dimensional volume including every combination of the various primary colorants available for a particular printing apparatus. Starting at the lower corner W of FIG. 1, a white value indicates no colorant is placed for a particular pixel in an image to be printed. Three (3) axes extend from the lower corner W. Numeric values are associated with points along each of the three axes. More specifically, the value of zero (0) is assigned to the lower corner W and the value of 255 is assigned to the point of full color saturation along the different directions. These three directions represent contributions of three primary colorants, yellow Y, cyan C, and magenta M. The more a particular colorant is apparent, the farther along any particular axis and, consequently, the higher the value (up to 255) of the particular color along the axis in the color space.
A color desired to be printed in an image is indicated as a source color at the location marked X. The color X is disposed near the inside surface of the center of the face within the gamut CS1 formed by the points marked C, W, M, B. Therefore, the color X is closest to the location in color space of grey LK. The particular problem addressed by the present invention occurs when selecting which colorants to use in order to obtain this desired source color X by a combination of, in this case, cyan C, grey LK, magenta M, light-cyan LC, and light-magenta LM.
Under the basic well known technique of error diffusion to select colorants, a source color X desired to be printed is located in color space, and the colorant selected for a particular pixel is the colorant which is closest, by Euclidean distance, to the source color X in color space. Once the particular colorant is selected, the distance, here indicated as d, is calculated in terms of both magnitude and direction, and then the error, which is the negative of the distance (same magnitude, opposite direction), is then diffused to influence the color selection decision for a certain subset of neighboring pixels, as is known in the art. In other words, a fixed proportion such as xe2x80x9cFloyd-Steinberg weightsxe2x80x9d of the error is applied to each of a plurality of the neighboring pixels, thereby altering the position of the source color for the neighboring pixels.
The contribution of the diffused error from the colorant selection in one pixel influences the color selection for a neighboring pixel. Notably, the colorant selection for the neighboring pixel may be different than it originally would have been because the error from a previous pixel selection has been added to it. The specific desired overall color (such as a shade of brown) is obtained as the cumulative result of this error diffusion over a reasonably large number of pixels. Of course, the present discussion is directed to obtaining a desired source color over a fairly large set of pixel areas. In xe2x80x9cbusyxe2x80x9d images, where the desired source color changes abruptly across the image, an error-diffusion technique will have a different effect on the selection of colorants.
When an intra-gamut colorant such as LK is selected by the error diffusion algorithm as the closest available color, the error, meaning the negative of the distance to the source color X, which is diffused to influence the selection of other pixels, occasionally causes a practical problem. When this error is added to the source color X for the selection of a colorant for a neighboring pixel, the resulting new source color, indicated as X= in FIG. 1, is pushed effectively out of the gamut CS1. This pushing of the new source color out of the gamut comes about because the selected colorant LK is located toward the center of the gamut, and thus the resulting error vector is pointed directly out of the gamut CS1. If this error-diffused new source color X= is the color that would serve as the new source color from which a colorant would be selected in the neighboring pixel, it would be impossible to select a combination of primary colors such as C, M, Y, etc. which could possibly simulate this color, and the whole error-diffusion process is compounded. A similar problem arises if the error diffusion algorithm chooses a color on one of the faces (e.g., LC+LM, LC+LY, LM+LY, LK+LC, LK+LM, and LK+LY) as the closest available color.
The present invention is directed to an error-diffusion system, specifically for use with an apparatus wherein intra-gamut colorants, such as grey, or lighter or diluted versions of primary color colorants, are available. Furthermore, the present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.
A method selects colorants, from a plurality of colorants defining a color space, for a set of pixels. The method obtains actual colors approximating desired colors for the respective pixels. The actual colors are output by an output apparatus. The method performs a variety of steps for each pixel in the set. Specifically, the method determines the desired color. Next, undercolor removal is performed for the desired color, thereby dividing the desired color into first and second components. A scalar error diffusion is performed for the first component of the desired color, resulting in a first component of the actual color. A vector error diffusion is performed for the second component of the desired color, resulting in a second component of the actual color. The vector error diffusion uses a reduced set of combinations achievable from the plurality of colorants. The first and second components of the actual color are combined to achieve the final actual color. The final actual color is output by the output apparatus.
In accordance with one aspect of the invention, the step of performing undercolor removal includes performing full undercolor removal.
In accordance with another aspect of the invention, the color space includes at least one colorant for outputting at least one level of a black. The step of performing undercolor removal includes identifying a level of black as the first component of the desired color and identifying respective levels of other colorants defining the color space as the second component of the desired color.
In accordance with a more limited aspect of the invention, the color space is defined by eight colorants. The determining step includes determining components of the eight colorants included in the desired color.
In accordance with even a more limited aspect of the invention, the color space includes at least two colorants for outputting respective levels of the black, at least two colorants for outputting a dark and a light level of at least two of a cyan, a magenta, and a yellow, and at least one colorant for outputting the other of the cyan, the magenta, and the yellow. The step of performing vector error diffusion includes selecting a closest achievable color not including two of the light levels.
In accordance with another aspect of the invention, the outputting step includes printing the actual color on a color printer.
One advantage of the present invention is that unavailable colorants either inside the color space or on the xe2x80x9cfacexe2x80x9d of the color space are not chosen for vector error diffusion.
Another advantage of the present invention is that artifacts, occurring from large errors when a vector error diffusion algorithm chooses these unavailable colorants, are decreased.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.